Some known schemes for improving bandwidth (BW) efficiency employ partial-response (PR) or correlative-coding, vestigial-sideband or quadrature vestigial-sideband (VSB/QVSB) transmission, and Quadrature-Quadrature Phase-Shift Keying (Q2PSK). In QVSB digital transmission, each of the in-phase and quadrature (i/q) baseline modulating channels contains the superposition of an independent pair of data streams.
QVSB embodies a very general approach to transmitting digital data in a bandwidth-efficient manner, and as a result, it is applicable to a wide variety of uses such as the Internet, Cellular, Personal Communications Service (PCS), Direct Broadcast Satellite (DBS), Cable Television (CATV), High-Definition Television (HDTV), etc. See, e.g., B. Henderson & J. Webb, “Quadrature Vestigial Sideband (QVSB) Data Transmission,” IEEE Transactions on Communications, Vol. COM-33, No. 12, pp. 1274–1280, December 1985, incorporated by reference. With respect to HDTV, which uses a VSB format, the QVSB technique enables transmission of two independent data streams on quadrature carriers, simultaneously—thereby doubling the transmission capacity relative to VSB. See, e.g., W. Sabin & E. Schoenike, Single-Sideband Systems and Circuits, New York, McGraw-Hill, 1995, incorporated by reference.
Although QVSB provides the advantage over VSB of doubling the information carrying capacity, the disadvantages are greater implementation complexity, and typically reduced detection noise-margins, due to crosstalk between the quadrature channels. In unraveling this crosstalk, the quadrature-crosstalk maximum likelihood-sequence-estimator (QC-MLSE) described in U.S. patent application Ser. No. 10/060,728 is sub-optimum because it operates on correlated noise inputs, while its algorithms are predicated on uncorrelated noise inputs. In particular, the Bit-Error-Ratio is typically degraded by 1.5 dB due to the effects of correlated noise. It would be desirable to be able to address the effect of correlated noise and thereby improve QVSB data detection.